In optical transmission systems it is necessary to multiplex signals of different wavelengths each modulated with data for transmission into a single main optical fiber known as a link or a transmission fiber, and vice versa. Such optical systems are said to be unidirectional when the different wavelengths propagate in a single direction along the link fiber and they are said to be bidirectional when the link fiber transmits a plurality of "go" wavelengths in one direction and a second plurality of different "return" wavelengths in the other direction. At present, bidirectional optical link systems are used in numerous applications such as telematics, telephone transmission, or teledistribution networks in which a central station communicates with peripheral stations constituted by subscriber transmitter-receiver apparatuses.
The Applicants' French patent application No. 83 07795 filed May 10, 1983 and entitled "Wavelength multiplexing and demultiplexing device using optical fibers" describes an optical multiplexer-demultiplexer having three wavelength transmission paths or channels using intermediate optical fibers in combination with a single link fiber. The three channels may be unidirectional, or there may be two go channels and one return channel. As described in this patent application, a three-channel unidirectional link has a link fiber which has a roof-shaped chamfer at each end, and three intermediate optical fibers each transmitting an optical signal of a given wavelength are optically coupled to each chamfered end of the link fiber. These wavelengths are multiplexed at one of the chamfered ends of the fiber by wavelength distribution in the link fiber, while they are demultiplexed at the other chamfered end of the fiber by selecting a wavelength by means of multi-layer dielectric interference filters deposited on the intermediate fibers. Likewise, for a bidirectional, three-channel link having two go channels and one return channel, the wavelengths are multiplexed and demultiplexed as before using two intermediate optical fibers coupled to each of the roff-shaped chamfered ends of the link fiber, with one of the intermediate fibers being bidirectional and each of the intermediate fibers including an interference filter.
However, such a three-channel multiplexer-demultiplexer device is very difficult to make in practice in that it is technologically difficult to chamfer the ends of an optical fiber into a roof-shape which is as optically perfect as possible. Consequently, the manufacturing cost of such a device is relatively high.
The aim of the present invention is to provide an optical multiplexer-demultiplexer in a simple and cheap manner, said multiplexer being essentially a three-channel device, and being achieved by coupling at least three intermediate fibers to the same end of a link fiber in a manner which is particularly satisfactory both from the optical point of view and from the mechanical point of view. This embodiment does not require complicated chamfering, is equally applicable to unidirectional and to bidirectional links, and is compact, i.e. it takes up little room.